Electric devices such as motors and generators are frequently classified according to the orientation of the lines of flux between the stationary and rotating elements. Thus, electrical generators and motors are often referred to as radial or axial field devices. Although radial field devices are in widespread use, axial field devices have been viewed as being of limited utility. The major reason for this is that heretofore axial field devices generally exhibited a low efficiency and required massive structure to achieve desired power capacities. Hence, radial field designs were generally preferred.
Prior art designs for axial field motors or the like as exemplified by U.S. Pat. No. 2,469,808, which issued to L. E. Aske on May 10, 1949, were generally of a pancake design and generally manifested low volumnmetric efficiency. Therefore, cores losses which include eddy current and hysteresis losses are substantial and greatly contribute to the overall inefficiency of the motor design. In U.S. Pat. No. 2,469,808, an open-face caged rotor induction motor of the pancake type is disclosed. The core of the motor comprises a flat annular ring formed from a tight, spirally wound strip of electrical steel ribbon. Other examples of motors of this type are disclosed in U.S. Pat. No. 3,591,819, which issued to Laing on July 6, 1979, and U.S. Pat. No. 2,356,972, which issued to Chubbuck on Aug. 29, 1944. Axial field motors exhibiting pancake design features generally manifested notoriously low volumnmetric efficiency and hence, high losses. Examples of axial field motors employing multiple rotor or stator designs are also set forth in U.S. Pat. No. 2,557,249 and 2,550,571 as issued to L. E. Aske on June 19, 1951 and B. Littman on Apr. 24, 1951, respectively.
The present invention proceeds upon a recognition and determination that marked improvements in the efficiency of axial field motors can be obtained by wholly rejecting the pancake-like design considerations prevailing in the prior art and instead adopting near square, optimized proportions so that by stacking rotors and stators, any desired geometry may be achieved without a substantial decrease in performance. Thus, in accordance with the present invention, the core element forming either the rotor or the stator, generally comprises a cylinder having an annular cross-section made of iron or the like. This core element is slotted and configured such that the sum of the slot widths per pole preferably equals approximately 60% of the axial length of the core element; the slot depth preferably equals approximately 60% of the axial length; and the total slot area (slot length.times.slot width.times.number of slots) preferably equals approximately 75% of the area of the iron cylinder less the area of the slots.
In addition to the marked efficiency improvement with core designs in accordance with the present invention, the weight of the core elements so designed and hence the overall structure of the resulting motor or generator may be minimized for a desired power capacity. As a result, a motor constructed in accordance with the teachings of the present invention will be lighter and hence more cost efficient than prior art axial field devices due to a marked reduction in material requirements.
Furthermore, while the core designs set forth herein may be formed of silicon, iron or similar conventional materials, efficiency is here additionally enhanced by markedly reducing hysteresis and eddy current losses by forming the core elements from a wound strip of an amorphous magnetic alloy rather than a wound strip of electrical steel as typically employed in the prior art. The high electrical resistivity of such amorphous magnetic alloys, which range from approximately 160 to 180 microhms/cm at 25.degree. C., results in a substantial decrease in eddy current losses, while the lower coercivity thereof markedly lowers hysteresis losses. In combination core losses exhibited by cores formed from amorphous magnetic alloys as employed herein have been found to be only one-seventh of those manifested by similarly configured cores formed of conventional materials. In this regard, it should be noted that while use of amorphous magnetic alloys has previously been suggested for the purpose of reducing losses in radial field devices, mechanical forming problems associated with the stampings required in radial field devices, have precluded the practical application of these materials to prevent such losses. Amorphous magnetic alloys are, however, readily available in strip or tape form which readily admit of winding to form the cylindrical cores employed within the instant invention.